The electricity generation by thermoelectricity is a technique utilizing Seebeck effects in semiconductor materials for realizing direct conversion from heat to electricity, which is characterized by long lifespan, high reliability, safe environment, etc. It has wide applications and potential social and economic effects in areas of electricity generation by photoelectricity and thermoelectricity, solar energy, and electricity generation by industrial waste heat. Improving the figure of merit of thermoelectric materials can be beneficial in improving energy conversion efficiency of thermoelectric electricity generation. Therefore, studies in the field of thermoelectric conversion focus on developing new thermoelectric materials with high performance. In another aspect, processes of researching and developing new thermoelectric material devices is of equal importance for improving energy conversion efficiency of thermoelectric electricity generation.
Thermoelectric devices mainly comprise two types of thermoelectric semiconductor elements, i.e. p-type and n-type. Since the voltage of a single thermoelectric device can be limited, electrodes are usually used to have a variety of p-type and n-type thermoelectric components connected in series for electrical conduction or connected in parallel for thermal conduction to construct a thermoelectric electricity generation module, thereby to acquire a higher voltage for selected applications. The choice of electrode materials and the combination thereof with thermoelectric elements are considerations in the manufacture of thermoelectric devices. Recently, Bi2Te3-based devices at a low temperature have been commercialized, commonly having copper as electrodes and adopting traditional soldering technique for welding. With respect to devices at an intermediate and high temperature, firstly, it is desirable that the coefficients of thermal expansion (CTE) of electrode material match with the thermoelectric material for choosing an electrode material, so as to reduce the thermal stress as much as possible during fabricating and using the devices, and to avoid failure in welding the electrode or invalidity during using the devices due to excessive thermal stress. Secondly, good electrical conductivities and thermal conductivities are desirable for the electrode material in order to reduce adverse impacts on device performance, such as energy conversion efficiency, due to the electric resistance and thermal resistance incurred by the electrode.
Filled skutterudite is regarded as a new thermoelectric material in an intermediate temperature with high performance, which has a promising future of application. A technique for welding Bi2Te3 devices is borrowed for welding electrodes on a low temperature side of a filled skutterudite device, where copper is chosen as the electrode material and the technique of tin soldering is adopted for welding. With respect to welding electrodes in a high temperature of filled skutterudite devices, according to existing reports, Cu, Mo, Ni—Cr, W, Ta, and their alloys, stainless steel (U.S. Pat. No. 6,005,182), Ag, Ag—Au, Ag—Cu, Fe (U.S. Pat. No. 6,759,586) and Nb (U.S. Pat. No. 6,563,039) are chosen as electrode materials, while copper brazing (U.S. Pat. No. 6,005,182, US 2002/0024154, CN 101114692, etc.), silver brazing (U.S. Pat. No. 6,759,586, US 2008/0023057, etc.), sintering (US 2006/0017170, U.S. Pat. No. 6,563,039, JP11195817, etc.) and the like are adopted as joining methods for electrodes and skutterudite materials.
Table 1 lists thermal expansion coefficients (CTEs), electrical conductivities and thermal conductivities of filled skutterudite and metallic materials. It can be seen that simple metals, except Ti, Fe, and Ni that have a CTE close to that of filled skutterudite, show large differences in CTE than filled skutterudite, while Ti, Fe and Ni exhibit much lower electrical conductivity and thermal conductivity than Cu, Mo, etc. Stainless steel, which mainly composes of Fe, Cr, Ni, etc., has a CTE closest to that of the filled skutterudite material. In addition, it is observed that Mo has a smaller CTE than the filled skutterudite material while Cu has a larger CTE than filled skutterudite. When Mo and Cu are combined into alloy, the alloy may have CTE close to that of filled skutterudite material by adjusting relative proportion of the two, and may also maintain good electrical conductivity and thermal conductivity of Cu and Mo. W and Cu are quite the same.
Recently, a commonly-adopted method for fabricating thermoelectric devices (for example, the method for fabricating thermoelectric devices recorded in Chinese patent application No. 200710044771.0) is mainly characterized by (i) first fabricating (sintering) a bulk element of a thermoelectric device in a die, (ii) welding an electrode at a high temperature onto the bulk element, (iii) welding the side at a low temperature with a ceramic plate by solders, and (iv) forming a π shaped device by cutting or the like. However, the existing method is not only complex, but also inevitably exposes thermoelectric materials (such as filled skutterudite) again to heat and pressure with a risk of degrading the performance of thermoelectric materials. Therefore, it is in urgent need of developing a new method for fabricating devices to simplify processing procedures and avoid adverse impacts on thermoelectric materials.
TABLE 1CTE, electrical conductivity, and thermal conductivity of materialsCTE (RT~875K)electrical conductivitythermal conductivityMaterial(×10−6 K−1)(×106Ω−1 m−1)(W/mK)CoSb3 based10-11P: 0.062-0.073P: 2.1-2.6skutteruditeN: 0.11-0.23(RT~850K)N: 2.2-3.0(RT~850K)Mo5.6-6.218.1138Cu18  59.6334W4.5-4.6(RT~100)18.9138Ti8.4-8.6(RT~100)1.921Ni13(RT~100)1682.8Fe12-13(RT~100)~437Ag19(RT~100)62.9429Ta6.5(RT~100)8.0357.5Nb7.2-7.3(RT~100)853.7stainless steel10-13(RT~100)1.5-2.514-16Mo50Cu5010.5~9.5 37.1230-270Mo70Cu308.9~8.522.3170-200WCu alloy9.054.3220-230